The guarding of large areas against intrusion always presents a considerable problem despite the many systems already developed for accomplishing this purpose. Especially in the case of international borders, the expense of installing an effective system is a major consideration because of the great lengths of such borders. Another persistent problem stems from the need to make sensitive detection means which do not at the same time deliver an excessive number of false alarms attributable to spurious stimulations, for instance, caused by induced fields from power lines or storms, by natural seismic activities, by the presence of wild animals, and by the aging and drifting of electrical components forming parts of the system.
U.S. Pat. No. 4,012,649 to Cook & Kerr issued Mar. 15, 1977, and assigned to Teledyne Industries, Inc., Geotech Division, the same assignee as the present application, teaches a buried piezoceramic stress/strain intrusion detector array in which individual detector units are spaced along an interconnecting cable, each of the detector units comprising a small brass box about one and half inches in diameter, and having at least one surface of the box which serves as a diaphragm supporting a piezoceramic transducer which is connected to the conductors of a cable passing through the box, the cable being sealed at entrance ferrules thereto. The cable with the detector units located at spaced intervals therealong is buried along a perimeter, for instance of an international border, and detects pressure from footsteps or from passing vehicle treads. This detector array has, however, the disadvantage of providing a number of false alarm signals, which false signals have been traced to the fact that the brass containers for the individual detector transducers were un-insulated on their outer surfaces, whereby electrical currents flowing through the earth can be conductively coupled into the cable, thereby introducing false readings which are attributable to non-intrusion phenomena rather than to mechanical stressing of the detector transducers within the metal containers. Considerable effort was made to insulate the outsides of the individual detector containers, but no flexible material was found which could be made to accurately conform to the shape of the detector containers while at the same time bonding to the cable jackets on either side thereof so as to achieve the necessary mechanical and insulating properties simultaneously. It was therefore necessary to develop a new detector construction which would provide about the same sensitivity to stressing of the soil in which the unit is buried, while at the same time providing complete detection insulation of the units from currents flowing through the earth.